The naval limitations treaties signed between
the World Wars forced warship designers and
strategists into a number of unusual expedients
that would not have otherwise existed. Ship
displacements and armament were dictated by
paper limits, not by the mission foreseen
for the ship, and so all sorts of warships
emerged from the shipyards either too small
or too large for their appointed tasks. And
in some cases, completely new ship types appeared.

Three
of Japan’s purpose-built seaplane carriers
appear in Second
World War at Sea: Strike South. They
are useful ships for the Japanese player,
but not nearly as capable as a small carrier
would be in their place.

The first ships to take airplanes to sea
were seaplane carriers, some years before
the first flight-deck carriers. In 1913 the
Imperial Japanese Navy converted the freighter
Wakamiya Maru (a prize taken during
the Russo-Japanese War) into a seaplane tender
with two Farman floatplanes. She operated
off the German colony of Tsingtao in August
and September 1914, until striking a mine
and limping back to Japan for repairs.

Wakamiya (she received warship
status in 1915 and thus lost the “Maru”
suffix) operated like most seaplane carriers
of the period, unloading her aircraft onto
the water for direct take-off under their
own power. Most therefore operated from harbors
or sheltered waters, but the Royal Navy and
the Imperial Russian Navy launched airstrikes
from the open sea by this method.

Japan operated one seaplane carrier for
most of the 1920s and 1930s, but in the mid-1930s
began converting several large freighters
to support seaplane operations along the Chinese
coast. At the same time, the “Secret
Program” began to lay down a number
of large auxiliaries, built to naval construction
standards, that could be easily converted
to light aircraft carriers during wartime.

These included the submarine support ships
Takasagi, Tsurugisaki and Taigei
and the passenger liners Kashiwara
Maru and Izumo Maru. And also
four large purpose-built seaplane carriers,
Chitose, Chiyoda, Mizuho and Nisshin.

Chitose at sea.

The big seaplane carriers had a cruiser’s
speed and a dual-purpose gun armament. Chitose,
Chiyoda and Mizuho had
four catapults and carried 24 aircraft; Nisshin
carried two catapults and 20 aircraft.
They had aircraft-handling, repair and service
facilities just like an aircraft carrier,
but still had to recover their aircraft off
the water. However, both the Washington and
London treaties classed seaplane carriers
as auxiliaries rather than warships, and thus
their tonnage did not count against Japan’s
total for true aircraft carriers.

Chitose and Chiyoda, laid
down in 1934 as part of the 1931 program,
displaced 11,000 tons and joined they fleet
in 1938. They had four 5-inch guns and their
diesels could drive them at 29 knots. The
almost-identical Mizuho had much
less powerful (but less expensive) machinery,
and could only make 22 knots. She carried
two more guns than the first pair. Nisshin
shared her hull design with the first
three, but reverted to more powerful machinery
and could make 28 knots. She had only two
catapults, but was fitted to carry and lay
700 mines.

Operationally, these ships would provide reconnaissance
capability, support for amphibious landings
and other secondary operations to free the
fleet’s carriers for more important
missions. In practice, they proved far less
useful than expected and within months of
the outbreak of war with the United States
they were being used as fast troop transports.

With seaplanes not as important as Japanese
planners had assumed, the four big seaplane
carriers went into the yards for a quick conversion.
Large doors were cut in their sterns to allow
them to launch midget submarines. Aircraft
capacity was halved, as a dozen small subs
took up the hangar space.

This mission also proved futile, as the
Americans refused to stay in place long enough
for the seaplane carriers to approach them
and launch the very short-ranged little submarines.
Despite great effort, the mini-submarine program
had very little success outside of a few harbor
strikes — and the mother ships for those
operations were large submarines, not surface
ships.

Nisshin off Bougainville,
firing at American scout plane.

Japan’s stunning defeat at Midway led
to activation of the “Secret Program”
for the seaplane carriers, the other five
ships having already been taken in hand for
conversion to aircraft carriers. Chitose
had been seriously damaged by American
aircraft during the Guadalcanal campaign while
operating as a fast transport, and this delayed
her reconstruction. Chiyoda began
rebuilding as a flush-decked light carrier
at Yokosuka Navy Yard in late 1942, and her
sister began work at Sasebo Navy Yard in early
1943.

Mizuho never made it to the yards,
falling victim to torpedoes from the American
submarine Drum in May 1942 while
off the coast of Japan. She would have required
new engines, apparently the same type of destroyer
boilers and turbines used to re-engine the
light carriers Zuiho and Shoho
when converted from submarine support
ships.

Nisshin was torpedoed in September
1942, but instead of heading straight to carrier
conversion was repaired and rushed into the
fight for Guadalcanal, where she sailed alongside
Chitose as a large, fast transport.
Unlike Chitose, she remained active
in this role into 1943; she was fast and with
her big cranes and large hangars she could
quickly unload 150mm artillery pieces, something
no requisitioned freighter or other warship
could accomplish. She was sunk by U.S. aircraft
while making a supply run from Rabaul to the
Shortland Islands in July 1943.

Under Japanese strategic thought of the
early 1930s, the first two purpose-built seaplane
carriers made a certain sense. Japan had not
yet withdrawn from the Washington and London
Naval Treaties, and the seaplane carriers
represented a possible force multiplier if
they could take scouting planes off carrier
decks. But by the time Mizuho was
laid down, the naval limitations treaties
had expired and Japan had decided not to attend
the second London conference. The Japanese
would have bene much better off laying down
fleet carriers in their place (a second Soryu
in place of Mizuho and another
Hiryu in place of Nisshin), and
ultimately these would have been cheaper to
construct than the seaplane carriers with
conversion expenses factored in (though the
air groups would have been much more costly).

In Strike South the seaplane carriers
(Chitose, Chiyoda, Mizuho plus three
merchant conversions) carry the E8N scout
plane. But the big ships also have the F1M
fighter, a biplane floatplane. While not a
very good combat plane in other theaters,
it’s adequate against the F2A Buffaloes
and other weak fighters put up by the British
and Dutch.